As is known, used or waste paper usually contains a certain proportion of unwanted stock or material which should be removed by processing for the purpose of the recycling of used paper. For this, a number of machines and processes are available to draw off as accepted stock the used paper, which is to be further processed, while the unwanted stock is removed therefrom as reject material. These reject materials are normally disposed of, or also, in special cases, reprocessed into new products. Since used paper processing normally takes place in aqueous suspension, the reject materials contain much water which makes them difficult to manipulate, and which considerably increases the transport and dumping costs for their disposal.
The mechanical separation of the water from the material by pressing has proved itself to be an economically viable and practical process. Consequently, for instance, worm presses are used in which a driven feed screw is arranged inside of a substantially concentric cylindrical or conical, perforated sheet metal jacket or the like. The reject material is fed in radially and is dewatered in cooperation with a dam apparatus, wherein the water can escape through the perforated sheet metal jacket or the like, whereas the reject materials are held back. The compression and dewatering of the reject can be improved when a conical form is chosen for the sheet metal jacket, the diameter of which reduces in the direction of movement of the reject material. Naturally, the outer worm diameter must also fit these geometrical dimensions. When the known dewatering worms are implemented as described, they are subject to a particularly large wear as a result of intensive rubbing between reject materials and the components of the machine. Namely, a considerable relative movement occurs under simultaneously enormous axial and radial forces between the already much thickened material and the components. Moreover, this reject material contains often many small metal particles and pieces of hard plastic, as well as fiber remnants which, as is known from practice, can lead to a high wear even of high quality metallic components.